1. Field of the Invention
This invention relates to an auto-focusing control system, and particularly to an automatic focusing device provided with focus detecting means for detecting the focus adjustment state of a lens system relative to an object and lens system control means for controlling the lens system to cause the lens system to be focused to the object on the basis of the output of the focus detecting means.
2. Description of the Prior Art
A device is known in which light-receiving elements capable of responding to the sharpness of an image are disposed, for example, forwardly and rearwardly of the predetermined focal plane of a lens system and the outputs of these light-receiving elements corresponding to the sharpness of the image are compared, whereby detection and control of the focus of the lens system are effected. In such a device, control of the focus is effected by detecting the magnitude relation between the outputs of the light-receiving elements, and for example, where the object lies at a great distance and the lens system is set to the in-focus position on the short distance side, or in the converse case or where the focal length of the lens system is relatively long and the focus thereof is deviated, the blur of the images on the two light-receiving elements becomes remarkably great, that is, the sharpness of the images becomes remarkably low and therefore, it becomes difficult to discriminate the focus and focusing control often becomes impossible.
This will hereinafter be described specifically. An auto-focusing control system is known in which, as shown, for example, in FIG. 1 of the accompanying drawing, light-receiving elements 3 and 4 capable of responding to the sharpness of the image are disposed at equidistant positions forward and rearward of the predetermined focal plane 2 of an objective lens 1 and the outputs of the light-receiving elements 3 and 4 corresponding to the sharpness of the image are compared to thereby discriminate the in-focus, the near focus and the far focus and the objective lens 1 is driven to the in-focus position, and in this system, where the output of the light-receiving element 3 corresponding to the sharpness of the image is greater than that of the light-receiving element 4 (the near focus state, i.e., the state indicated by dotted line in FIG. 2 of the accompanying drawings), the objective lens 1 is driven in a direction for inwardly displacing the objective lens 1, namely, toward the long distance in-focus position side and conversely, where the output of the light-receiving element 4 corresponding to the sharpness of the image is greater than that of the light-receiving element 3 (the far focus state, namely, the state indicated by solid line in FIG. 2), the objective lens 1 is driven in a direction for outwardly displacing the objective lens 1, namely, toward the short distance in-focus position side, and the objective lens 1 is stopped at the infocus position (the state of FIG. 1) whereat the outputs of the light-receiving elements 3 and 4 corresponding to the sharpness of the image become substantially equal to each other.
In the case of an ideal lens system and electric system, even if the lens system is greatly deviated from the in-focus position, when the lens system is in the far focus state, the output of the light-receiving element 4 is considered to be greater than that of the light-receiving element 3, but in the actual system, when the lens system is greatly deviated from the in-focus position due to the noise of the electric system or the aberration of the lens system, particularly, creation of false resolution, the levels of the outputs of the light-receiving elements are sometimes inverted as indicated by a portion a FIG. 3 of the accompanying drawings. Therefore, the range in which an effective focus detection signal is obtained, that is, the focal-point detectable area of focus detecting means, is limited as indicated between A-B of FIG. 3. If such a limitation is present, where the lens system is positioned outside, the region between A-B, detection of the focus becomes impossible and therefore, control of the lens system also becomes impossible, and this has led to the disadvantage that the state becomes equivalent to the in-focus state in terms of signal and therefore the lens system remains stopped at a wrong position.
Discrimination between the true in-focus state and the false in-focus state which occurs when the focus is greatly deviated can be accomplished at least by a method as will hereinafter be described. That is, as described above, in the in-focus state and incondition in which the focus is greatly deviated, the outputs of the two light-receiving elements become equal to each other, but in the in-focus state, both of the outputs of the two light-receiving elements are above a certain level and accordingly, when the outputs of the two light-receiving elements have become equal to each other, discrimination between the in-focus state and the condition in which the focus is greatly deviated can be accomplished by discriminating whether the output of each of the light-receiving element is above a certain level. Besides this, the above-described discrimination is possible by disposing, for example, a third light-receiving element at the predetermined focal plane position and making such a design that when the outputs of the two light-receiving elements forward and rearward of the predetermined focal plane have become equal to each other, it is discriminated whether the output of the third light-receiving element is above a certain level.
However, these methods only enable the discrimination between the in-focus state and the condition in which the focus is greatly deviated to be accomplished and cannot achieve a drastic solution to the disadvantage that when the focus is greatly deviated, the lens system is stopped at that position and the system cannot get out of such stopped position by its ability.